Early development of the somatotopic map and barrel patterning in rat somatosensory cortex

Patterned activity via spinal dorsal quadrant inputs is necessary for the formation of organized somatosensory maps

The normal development of the somatosensory system requires intact sensory inputs from the periphery during a critical window of time early in development. Here we determined how the removal of only part of the ascending spinal inputs early in development affects the anatomical and neurophysiological development of the somatosensory system. We performed spinal overhemisections in rat pups at C3/C4 levels on the third day after birth. This procedure hemisects the spinal cord on one side and transects the dorsal funiculus on the other side. When the rats were 6-8 months old, the responsiveness and somatotopy of the primary somatosensory cortex (S1) contralateral to the hemisection were determined using standard multiunit mapping techniques. Sections of the flattened cortex were processed for cytochrome oxidase activity, Nissl substance, or myelin. We found that histologically apparent modules that are normally present in the regions of the forepaw and the hindpaw representations were absent, whereas the lateral barrel field representing the face was completely normal. The neurons in the forepaw regions of S1 either did not respond to the stimulation of the skin of any region of the body or responded to the stimulation of the upper arm afferents that enter the spinal cord rostral to the site of the lesion. The results show that a lack of normal sensory inputs via ascending pathways in the dorsal spinal cord during early development results in massive anatomical and neurophysiological abnormalities in the cortex. Intact crossed spinothalamic pathways are unable to support the normal development of the forepaw barrels.

Role of development in reorganization of the SI forelimb-stump representation in fetally, neonatally, and adult amputated rats

Rats that sustain forelimb removal on postnatal day (P) 0 exhibit numerous multi-unit recording sites in the forelimb-stump representation of primary somatosensory cortex (SI) that also respond to hindlimb stimulation when cortical GABAA+B receptors are blocked. Most of these hindlimb inputs originate in the medial SI hindlimb representation. Although many forelimb-stump sites in these animals respond to hindlimb stimulation, very few respond to stimulation of the face (vibrissae or lower jaw), which is represented in SI just lateral to the forelimb. The lateral to medial development of SI may influence the capacity of hindlimb (but not face) inputs to "invade" the forelimb-stump region in neonatal amputees. The SI forelimb-stump was mapped in adult (>60 days) rats that had sustained amputation on embryonic day (E) 16, on P0, or during adulthood. GABA receptors were blocked and subsequent mapping revealed increases in nonstump inputs in E16 and P0 amputees: fetal amputees exhibited forelimb-stump sites responsive to face (34%), hindlimb (10%), and both (22%); neonatal amputees exhibited 10% face, 39% hindlimb, and 5% both; adult amputees exhibited 10% face, 5% hindlimb, and 0% both, with approximately 80% stump-only sites. These results indicate age-dependent differences in receptive-field reorganization of the forelimb-stump representation, which may reflect the spatiotemporal development of SI. Results from cobalt chloride inactivation of the SI vibrissae region and electrolesioning of the dysgranular cortex suggest that normally suppressed vibrissae inputs to the SI forelimb-stump area originate in the SI vibrissae region and synapse in the dysgranular cortex.

N-cadherin regulates ingrowth and laminar targeting of thalamocortical axons

Thalamocortical axons are precisely targeted to cortical layer IV, but the identity of specific molecules that govern the establishment of laminar specificity in the thalamocortical projection has been elusive. In this study, we test the role of N-cadherin, a homophilic cell adhesion molecule, in laminar targeting of thalamocortical axons using cocultured thalamic and cortical slice explants exposed to N-cadherin function-blocking antibodies or inhibitory peptides. In untreated cocultures, labeled thalamocortical axons normally grow to and stop in layer IV, forming terminal-like arbors. In the N-cadherin-blocked cocultures, thalamic axons reach layer IV by growing through deep layers at the same rate as those in the untreated cocultures, but instead of terminating in layer IV, they continue growing uninterruptedly through layer IV and extend into supragranular layers to reach the outermost cortical edge, where some form terminal-like arbors in this aberrant laminar position. In cocultures in which the cortical slice is taken at an earlier maturational stage, one that corresponds to a time when thalamic axons are normally growing through deep layers before the emergence of layer IV from the cortical plate, thalamic axon ingrowth through deep layers is significantly attenuated by N-cadherin blocking reagents. These data indicate that N-cadherin has multifaceted roles in establishing the thalamocortical projection, governing aspects of both thalamic axon ingrowth and laminar targeting by acting as a layer IV stop signal, which progressively change in parallel with the maturational state of the cortex.

Prenatal development of neural excitation in rat thalamocortical projections studied by optical recording

To elucidate the formation of early thalamocortical synapses we recorded optical images with voltage-sensitive dyes from the cerebral cortex of prenatal rats by selective thalamic stimulation of thalamocortical slice preparations. At embryonic day (E) 17, thalamic stimulation elicited excitation that rapidly propagated through the internal capsule to the cortex. These responses lasted less than 15 ms, and were not affected by the application of glutamate receptor antagonists, suggesting that they might reflect presynaptic fiber responses. At E18, long-lasting (more than 300 ms) responses appeared in the internal capsule and in subplate. By E19, long-lasting responses increased in the cortical subplate. By E21, shortly before birth, the deep cortical layers were also activated in addition to the subplate. These long-lasting responses seen in the internal capsule and subplate were blocked by the antagonist perfusion, but the first spike-like responses still remained. The laminar location of the responses was confirmed in the same slices by Nissl staining and subplate cells were labeled by birthdating with bromodeoxyuridine at E13. Our results demonstrate that there is a few days delay between the arrival of thalamocortical axons at the subplate at E16 and the appearance of functional thalamocortical synaptic transmission at E19. Since thalamocortical connections are already functional within the subplate and in the deep cortical plate at embryonic ages, prenatal thalamocortical synaptic connections could influence cortical circuit formation before birth.

Characterizing the functional significance of the neonatal rat vibrissae prior to the onset of whisking

The present series of experiments assessed how information from the whiskers controls and modulates infant rat behavior during early learning and attachment. Passive vibrissal stimulation can elicit behavioral activity in pups throughout the first two postnatal weeks, although orienting to the source of stimulation is evident only after ontogenetic emergence of whisking. In addition, while pups were capable of demonstrating learning in a classical conditioning paradigm pairing vibrissa stimulation with electric shock, no corresponding changes were detected in the anatomy of the barrel cortex as determined by cytochrome oxidase (CO) staining. Finally, the role of whiskers in a more naturalistic setting was determined in postnatal day (PN)3-5 and PN11-12 pups. Our results showed that both nipple attachment and huddling were disrupted in whisker-clipped PN3-5 pups but only marginally altered in PN1I 1-12 pups. Together, these results suggest that the neonatal whisker system is behaviorally functional and relevant for normal mother-infant interactions, though it lacks the sophistication of a mature whisker system that evokes very specific and directed responses.

The early topography of thalamocortical projections is shifted in Ebf1 and Dlx1/2 mutant mice

The prevailing model to explain the formation of topographic projections in the nervous system stipulates that this process is governed by information located within the projecting and targeted structures. In mammals, different thalamic nuclei establish highly ordered projections with specific neocortical domains and the mechanisms controlling the initial topography of these projections remain to be characterized. To address this issue, we examined Ebf1(-/-) embryos in which a subset of thalamic axons does not reach the neocortex. We show that the projections that do form between thalamic nuclei and neocortical domains have a shifted topography, in the absence of regionalization defects in the thalamus or neocortex. This shift is first detected inside the basal ganglia, a structure on the path of thalamic axons, and which develops abnormally in Ebf1(-/-) embryos. A similar shift in the topography of thalamocortical axons inside the basal ganglia and neocortex was observed in Dlx1/2(-/-) embryos, which also have an abnormal basal ganglia development. Furthermore, Dlx1 and Dlx2 are not expressed in the dorsal thalamus or in cortical projections neurons. Thus, our study shows that: (1) different thalamic nuclei do not establish projections independently of each other; (2) a shift in thalamocortical topography can occur in the absence of major regionalization defects in the dorsal thalamus and neocortex; and (3) the basal ganglia may contain decision points for thalamic axons' pathfinding and topographic organization. These observations suggest that the topography of thalamocortical projections is not strictly determined by cues located within the neocortex and may be regulated by the relative positioning of thalamic axons inside the basal ganglia.

Developmental patterns of cadherin expression and localization in relation to compartmentalized thalamocortical terminations in rat barrel cortex

The wiring of synaptic circuitry during development is remarkably precise, but the molecular interactions that enable such precision remain largely to be defined. Cadherins are cell adhesion molecules hypothesized to play roles in axon growth and synaptic targeting during development. We previously showed that N-cadherin localizes to ventrobasal (VB) thalamocortical synapses in rat somatosensory (barrel) cortex during formation of the whisker-map in layer IV (Huntley and Benson [1999] J. Comp. Neurol. 407:453-471). Such specific association of N-cadherin with one identified afferent pathway raises the prediction that other cadherins are expressed in barrel cortex and that these are, in some combination, also differentially associated with distinct inputs. Here, we first show that N-cadherin and three other classic cadherins (cadherin-6, -8, and -10) are expressed contemporaneously in barrel cortex with relative levels of postnatal expression that are highest during the first 2 weeks, when afferent and intrinsic circuitries are forming and synaptogenesis is maximal. Each displayed distinct, but partly overlapping laminar patterns of expression that changed over time. Cadherin-8 probe hybridization formed a particularly striking pattern of intermittent, columnar patches extending from layer V through layer III, which was first detectable at approximately postnatal day 3. The patches were centered precisely over regions of dysgranular layer IV and, in the whisker barrel field, over barrel septa. This pattern is similar to that formed by the terminal distribution of thalamocortical afferents arising from the posterior nucleus (POm), suggesting cadherin-8 association with the POm thalamocortical synaptic circuit. Consistent with this, cadherin-8 mRNAs were enriched in the POm nucleus, and cadherin-8 immunolabeling in layer IV was enriched in barrel septa and codistributed with labeled POm thalamocortical synaptic-like puncta. The striking molecular parcellation of at least two different cadherins to the two, converging thalamic pathways that terminated in non-overlapping barrel center and septal compartments in layer IV strongly suggested that cadherins provide requisite molecular recognition and targeting that enable precise construction of thalamocortical and other synaptic circuitry.

Refinement of thalamocortical arbors and emergence of barrel domains in the primary somatosensory cortex: a study of normal and monoamine oxidase a knock-out mice

In the rodent primary somatosensory cortex, the thalamocortical axons (TCAs) are organized into clusters that correspond to functional units in the periphery. Around these axons, neurons in layer IV aggregate as barrels. To understand how this organization emerges, we analyzed TCA development in mice that do not form barrels, the monoamine oxidase A knock-out (MAOA-KO), and in MAOA/5-HT(1B) receptor double-KO mice, which have a restored barrel field. We show that TCAs already attain cortical layer IV on the day of birth. They are uniformly distributed in this layer from postnatal day 0 (P0) to P2 and secondarily coalesce into barrel domains in layer IV, over a 3 d period (P3-P5), with no prepatterning in the deeper layers. In MAOA-KO mice, the uniform distribution of the TC projection is maintained, and no axon clusters emerge. Individual TCA arbors were traced after carbocyanine injections. At P1, TCAs were poorly branched and covered variable tangential widths, encompassing one to two prospective barrels. At P7 the number of TCA branches increased 10-fold in layer IV and became restricted to one barrel. In MAOA-KO mice, there was a 50% reduction of the TCA terminal branches in layer IV, with a 40% increase in their tangential extent. These defects were corrected in the MAOA/5-HT(1B) double knock-out mice, indicating an effect of the presynaptic 5-HT(1B) receptor on axon branching. Our results indicate that the barrel-deficient phenotype of MAOA-KO mice results from an altered refinement of the TCA arbors in their target layer IV, involving branch elaboration and collateral retraction during early postnatal life.

Postnatal refinement of auditory nerve projections to the cochlear nucleus in cats

Studies of visual system development have suggested that competition driven by activity is essential for refinement of initial topographically diffuse neuronal projections into their precise adult patterns. This has led to the assertion that this process may shape development of topographic connections throughout the nervous system. Because the cat auditory system is very immature at birth, with auditory nerve neurons initially exhibiting very low or no spontaneous activity, we hypothesized that the auditory nerve fibers might initially form topographically broad projections within the cochlear nuclei (CN), which later would become topographically precise at the time when adult-like frequency selectivity develops. In this study, we made restricted injections of Neurobiotin, which labeled small sectors (300-500 microm) of the cochlear spiral ganglion, to study the projections of auditory nerve fibers representing a narrow band of frequencies. Results showed that projections from the basal cochlea to the CN are tonotopically organized in neonates, many days before the onset of functional hearing and even prior to the development of spontaneous activity in the auditory nerve. However, results also demonstrated that significant refinement of the topographic specificity of the primary afferent axons of the auditory nerve occurs in late gestation or early postnatal development. Projections to all three subdivisions of the CN exhibit clear tonotopic organization at or before birth, but the topographic restriction of fibers into frequency band laminae is significantly less precise in perinatal kittens than in adult cats. Two injections spaced > or = 2 mm apart in the cochlea resulted in labeled bands of projecting axons in the anteroventral CN that were 53% broader than would be expected if they were proportional to those in adults, and the two projections were incompletely segregated in the youngest animals studied. Posteroventral CN (PVCN) projections (normalized for CN size) were 36% broader in neonates than in adults, and projections from double injections in the youngest subjects were nearly fused in the PVCN. Projections to the dorsal division of the CN were 32% broader in neonates than in adults when normalized, but the dorsal CN projections were always discrete, even at the earliest ages studied.

Complementary expression and heterophilic interactions between IgLON family members neurotrimin and LAMP

Neurotrimin (Ntm) and the limbic system-associated membrane protein (LAMP) are members of the IgLON (LAMP, OBCAM, Ntm) family of glycorylphosphatidylinositol anchored neural cell adhesion molecules. We previously reported that LAMP and Ntm promote adhesion and neurite outgrowth via a homophilic mechanism, suggesting that these proteins promote the formation of specific neuronal circuits by homophilic interactions. In this report, we have further characterized the expression and binding specificity of Ntm. Using a newly generated monoclonal antibody to Ntm, we demonstrated that this protein is largely expressed in a complementary pattern to that of LAMP in the nervous system, with co-expression at a few sites. Ntm is expressed at high levels in sensory-motor cortex and, of particular note, is transiently expressed in neurons of cortical barrel fields and corresponding thalamic "barreloids." Binding of a recombinant, soluble form of Ntm to CHO cells expressing either Ntm or LAMP demonstrates that Ntm and LAMP interact both homophilically and heterophilically. In contrast to conventional growth-promoting activity of Ig superfamily members, LAMP strongly inhibits the outgrowth of Ntm-expressing dorsal root ganglion (DRG) neurons in a heterophilic manner. These anatomical and functional data support the concept that homophilic and heterophilic interactions between IgLON family members are likely to play a role in the specification of neuronal projections via growth promoting and inhibiting effects, respectively.

Cholinergic depletion by IgG192-saporin retards development of rat barrel cortex

This study examines the role of cholinergic projections from the basal forebrain on development of the rodent barrel cortex. Pups were administered the immunotoxin IgG192-saporin (0.1 microg) intraventricularly at postnatal day (P) 0 and sacrificed at P1-P7. One ventricle was injected with saporin while the other side received saline, allowing comparison between the two sides of the same animal, as well as with controls receiving saline only. Compared to control animals, neuronal loss in the basal forebrain was present on both sides of saporin-treated pups but was significantly greater on the toxin-treated side, in all age groups and regions sampled. Depletion of acetylcholine did not prevent the formation of the barrel pattern, however it delayed its emergence by approximately 1 day. At P4, the thickness of layer IV barrel cortex was also significantly reduced; this reduction was undetectable by P7. From P3 to P5, the ratios of intensity of staining for acetylcholinesterase between the barrel centres and septa on the toxin-treated side were significantly lower than those on the saline side, although normal densities were present by P7. Thus, the depletion of cholinergic innervation at birth causes a transient delay in the development of the barrel pattern during the first postnatal week.

Synaptic patterns of thalamocortical afferents in mouse barrels at postnatal day 11

This study focuses on the synaptic output patterns of thalamocortical axons in mouse barrel cortex at postnatal day (P) 11. Axons were labeled by biotinylated dextran amine transported anterogradely following injection in vivo into the ventrobasal thalamus. Labeled axons in the posteromedial barrel subfield were examined by light and electron microscopy and then reconstructed in three dimensions to assess the spatial distribution of their synapses. Thalamocortical axons form asymmetrical synapses, both at varicosities and along cylindrical portions of the axons; usually, only one synapse occurs per site, contrasting with the case in the adult, in which multiple synapses are typical. At P11, varicosities without synapses are common. As in adult barrels, approximately 80% of synapses formed by thalamocortical axons are with dendritic spines; 20% are with dendritic shafts. The similarity in the distribution of thalamocortical synapses onto spines vs. dendrites in developing and mature barrels indicates that adult synaptic patterns already are specified at a very early stage of thalamocortical synaptogenesis.

Radial migration in the cerebral cortex is enhanced by signals from thalamus

The six layered cerebral cortex derives from cells that divide in the ventricular zone and migrate to their final destination in the cortical plate (future cortex). In the mouse, cortical layer III and IV neurons undergo their final mitotic division at around E16, at which time thalamic axons are beginning to enter the cortex. We used bromodeoxyuridine-birth dating of cells in cortical slice cultures to show that the thalamus enhances the migration out of the ventricular zone of future layer III/IV cells. When cortical slices were cultured alone, less than 35% of cells born in vitro on E16 were present in the pial half of the slice after 48 h in culture. In contrast, when cortical slices were cocultured with thalamus, 69% of these cells were found in the pial half of the slice. Explants of other developing tissues did not mimic the effect of the thalamus. The thalamus had no obvious effect on cortical radial glial cells, cortical cell viability or maintenance of cortical slice structure. We found that most precursors born at a similar age but in vivo, shortly before cortical slices were isolated, migrated to the pial half of the cultured slices in the absence of a cocultured thalamic explant. Thus, E16 cortical slices cultured without thalamus permit migration of cells born in vivo and therefore already exposed to the thalamus. Our results indicate that the thalamus provides factors to E16-born cortical precursors that enhance their directed migration out of the ventricular zone to the cortical plate.

Whisker-related neural patterns develop normally despite severe whisker defects in Msx2 knockout mice

In mice, whiskers on the snout form a highly specialized tactile organ with exquisitely patterned neural representations in the brain. Targeted deletion of the Msx2 gene leads to severe craniofacial defects, and stubby, curly whiskers. We examined the whisker pad histology, innervation, and whisker-related pattern formation along the trigeminal pathway in Msx2 -/- mice. Although the whiskers are severely deformed, whisker follicle structure, pattern and density of innervation, as well as central neural patterns in the brainstem, thalamus, and cortex appeared normal. We conclude that whisker-related neural patterns can form in the absence of normal whiskers, as long as whisker follicle innervation is intact.

Neural activity: sculptor of 'barrels' in the neocortex

A major portion of the primary somatosensory cortex of rodents is characterized by the discrete and patterned distribution of thalamocortical axons and layer IV granule cells ('barrels'), which correspond to the spatial distribution of whiskers and sinus hairs on the snout. In recent years several mutant mouse models began unveiling the cellular and molecular mechanisms by which these patterns emerge presynaptically and are reflected postsynaptically. Neural activity plays a crucial role in conferring presynaptic patterns to postsynaptic cells via neurotransmitter receptor-mediated intracellular signals. Here we review recent evidence that is finally opening the doors to understanding the cellular and molecular mechanisms of pattern formation in the neocortex.

Rapid development and plasticity of layer 2/3 maps in rat barrel cortex in vivo

Cortical synaptic circuitry develops rapidly in the second postnatal week, simultaneous with experience-dependent turnover of dendritic spines. To relate the emergence of sensory maps to synaptogenesis, we recorded synaptic potentials evoked by whisker deflection in layer 2/3 neurons from postnatal day (P) 12 to 20. At P12, synaptic responses were undetectable. Only 2 days later in life (P14), receptive fields had mature organization. Sensory deprivation, if initiated before P14, disrupted receptive field structure. In layer 4, responses and maps were already mature by P12 and insensitive to deprivation, implying that barrel cortex develops from layer 4 to layer 2/3. Thus, P12-14 is a critical period shared by layer 2/3 synapses and their spines, suggesting that spine plasticity is involved in the refinement of maps.

Neither peripheral nerve input nor cortical NMDA receptor activity are necessary for recovery of a disrupted barrel pattern in rat somatosensory cortex

Elevating cortical serotonin (5-HT) in rats from postnatal day (P-) 0 to P-6 by administering the monoamine oxidase (MAO(A)) inhibitor, clorgyline, produces a dose-dependent spectrum of effects on rat somatosensory organization, ranging from enlarged with indistinct septa to a complete lack of vibrissae-related patterns. However, if clorgyline treatment is stopped on P-6, a qualitatively and quantitatively normal vibrissae-related pattern of thalamocortical afferents appears in somatosensory cortex (S-I) on P-10. We employed high performance liquid chromatography (HPLC), infraorbital nerve (ION) transection, N-methyl-D-aspartate (NMDA) receptor blockade, 1,1'-dioctadecyl-3,3,3"3'-tetramethylindocarbocyanine perchlorate (DiI) labeling of thalamic afferents, and CO histochemistry to determine whether peripheral nerve input and/or cortical NMDA receptor activity were required for the recovery of vibrissae-related patterns in clorgyline-treated animals. Clorgyline administration from P-0 to P-6 produced a 1589.4+/-53.3% increase in cortical 5-HT over control animals on P-6 and a 268.8+/-6.3% elevation over controls at P-10. Postnatal day 6 pups had significantly altered vibrissae-related patterns in S-I following 6 days of clorgyline treatment but by P-10, the characteristic vibrissae-related patterns were restored. Neither transection of the ION nor application of the NMDA antagonist, DL-2-amino-5-phosphonovaleric acid (APV), to the cortices of P-6 pups that were treated with clorgyline from birth had any significant effect on the recovery of the vibrissae-related patterns by P-10. These results indicate that neither peripheral nerve input nor cortical NMDA receptor activity are necessary for the restoration of cortical vibrissae-related patterns in rats that have sustained transient elevations of 5-HT.

Morphology and growth patterns of developing thalamocortical axons

It is increasingly evident that the actions of guidance factors depend critically on the cellular and molecular context in which they operate. For this reason we examined the growth cone morphology and behavior of thalamic fibers in the relatively natural environment of a slice preparation containing the entire pathway from thalamus to cortex. Axons were labeled with DiI crystals and imaged with a laser-scanning confocal microscope for up to 8 hr. Their behavior was analyzed in terms of morphology, extension rates, shape of trajectory, frequency of branching, and percentage of time spent in advance, pause, and retraction. Thalamic fibers had distinct and stereotyped growth patterns that related closely to their position; within the striatum growth cones were small and elongated, rarely extending filopodia or side branches. Axons grew quickly, in straight trajectories, with minimal pauses or retractions. When they reached the ventral intermediate zone, axons slowed down, often coming to a complete stop for up to several hours, and their growth cones became larger and more complex. During pauses there were continuous extensions and retractions of filopodia and/or side branches. When advance resumed, it was often to a different direction. These results demonstrate consistent regional variations in growth patterns that identify an unexpected decision region for thalamic axons. They provide the basis for examining the roles of guidance cues in an accessible yet intact preparation of the thalamocortical pathway and allow for an evaluation of previously suggested pathfinding mechanisms.

Neonatal lead exposure impairs development of rodent barrel field cortex

Childhood exposure to low-level lead can permanently reduce intelligence, but the neurobiologic mechanism for this effect is unknown. We examined the impact of lead exposure on the development of cortical columns, using the rodent barrel field as a model. In all areas of mammalian neocortex, cortical columns constitute a fundamental structural unit subserving information processing. Barrel field cortex contains columnar processing units with distinct clusters of layer IV neurons that receive sensory input from individual whiskers. In this study, rat pups were exposed to 0, 0.2, 1, 1.5, or 2 g/liter lead acetate in their dam's drinking water from birth through postnatal day 10. This treatment, which coincides with the development of segregated columns in the barrel field, produced blood lead concentrations from 1 to 31 microg/dl. On postnatal day 10, the area of the barrel field and of individual barrels was measured. A dose-related reduction in barrel field area was observed (Pearson correlation = -0.740; P < 0.001); mean barrel field area in the highest exposure group was decreased 12% versus controls. Individual barrels in the physiologically more active caudoventral group were affected preferentially. Total cortical area measured in the same sections was not altered significantly by lead exposure. These data support the hypothesis that lead exposure may impair the development of columnar processing units in immature neocortex. We demonstrate that low levels of blood lead, in the range seen in many impoverished inner-city children, cause structural alterations in a neocortical somatosensory map.

Dopamine D(3) receptor is selectively and transiently expressed in the developing whisker barrel cortex of the rat

The rodent primary somatosensory cortex (SI) contains a map of the body surface, the most conspicuous part of which are "barrels," neuronal aggregates in layer IV that receive somatotopic projections from whiskers on the rodent's snout. We report that the D(3) dopamine receptor (D(3)R) is selectively and transiently expressed in SI during the first 2 weeks of postnatal development. D(3)R binding sites and mRNA overlap completely and are limited to layer IV of SI. D(3)R/mRNA are organized in a pattern corresponding to somatotopic representations of the body (e.g., whiskers, jaws, paws, etc.) with the highest expression in the barrel field. D(3) mRNA is first detected at postnatal day (P)4, increases rapidly until P7-10, and sharply decreases after P14. D(3)R binding sites are detectable at P6, peak at P14, and decline afterwards. D(1), D(2), D(4), or D(5) mRNAs display dissimilar expression pattern. D(1) mRNA is mostly confined to infragranular layers throughout the cortex. D(4) mRNA expression in layer IV rises by 4 weeks postnatal, when D(3)R expression is virtually undetectable. Quantitative analysis of D(3) mRNA expression demonstrates that the proportion of D(3) mRNA-positive cells decreases between P7 and P14, whereas mRNA concentration per cell remains stable. Moreover, D(3)R number continues to rise, whereas mRNA levels begin to decline. Thus, a process limiting D(3)R expression to fewer cells may occur that also induces changes in post-transcriptional regulation of D(3)R expression in remaining cells. These findings indicate that dopamine acting via D(3)R may play an important role in the development or function of the SI.

Consistency behind trial-to-trial variation in intrinsic optical responses to single-whisker movement in the rat D1-barrel cortex

We examined consistent characteristics behind the trial-to-trial variati on in intrinsic optical imaging of single barrel cortical responses to D 1-whisker movement in 2-5-week postnatal (2-5 W) and adul t (>9-weeks) Wistar rats, and we identified the effective are a of the neural response. The extent/size, configuration and orientation of the intrinsic optical response area varied from trial-to-trial with the same whisker stimulation. We argue that the trial-to-trial variation was due to cortical blood circulation related to the barrel neural activity. Subsequently, interpolating a family of the traces of the optical response area imaged with repeated stimulation for each animal, we extracted a centered circular area from the trial-to-trial response for each animal. Although the trial-to-trial variation decreased gradually with age, the spatial extent of the interpolated response area was consistently about 660 microm in diameter, in agreement with that measured morphologically and/or histochemically. A possible interpretation is that the optically defined area appears to image the actual effective single-barrel response area, as a first approximation. Furthermore, the constancy of the extracted area independent of age suggests that the barrel cortex is, in fact, virtually mature by 2 weeks of age. The extracted area was also nearly independent of the frequency (>/=5 Hz) of whisker movement.

Vibrissae-evoked behavior and conditioning before functional ontogeny of the somatosensory vibrissae cortex

The following experiments determined that the somatosensory whisker system is functional and capable of experience-dependent behavioral plasticity in the neonate before functional maturation of the somatosensory whisker cortex. First, unilateral whisker stimulation caused increased behavioral activity in both postnatal day (P) 3-4 and P8 pups, whereas stimulation-evoked cortical activity (14C 2-deoxyglucose autoradiography) was detectable only in P8 pups. Second, neonatal rat pups are capable of forming associations between whisker stimulation and a reinforcer. A classical conditioning paradigm (P3-P4) showed that the learning groups (paired whisker stimulation-shock or paired whisker stimulation-warm air stream) exhibited significantly higher behavioral responsiveness to whisker stimulation than controls. Finally, stimulus-evoked somatosensory cortical activity during testing [P8; using 14C 2-deoxyglucose (2-DG) autoradiography] was assessed after somatosensory conditioning from P1-P8. No learning-associated differences in stimulus-evoked cortical activity were detected between learning and nonlearning control groups. Together, these experiments demonstrate that the whisker system is functional in neonates and capable of experience-dependent behavioral plasticity. Furthermore, in contrast to adult somatosensory classical conditioning, these data suggest that the cortex is not required for associative somatosensory learning in neonates.

Altered spatial patterns of functional thalamocortical connections in the barrel cortex after neonatal infraorbital nerve cut revealed by optical recording

In rodents, the somatosensory cortex has a cell aggregation cluster termed the barrel, reflecting a whisker vibrissa, and this barrel formation is disrupted by infraorbital nerve cut at birth. In the present study, we prepared thalamocortical slice preparations from rats that received infraorbital nerve cut either at birth or at postnatal day (P) 7 and those from normal rats, recorded the optical response reflecting neural excitation in the somatosensory cortex with a voltage-sensitive dye (RH482) and compared the optical responses from lesioned rats with those from normal rats. In normal rats at P10, the optical response elicited electrically by thalamic stimulation propagated to the cortex, and then several patchy clusters appeared in layer IV. The size and location of these patchy responses precisely matched either barrels identified by cytochrome oxidase staining or terminal arbors of thalamocortial axons stained with biotinylated dextran amine. In contrast, at P10 in P0-lesioned rats, clusters having a wider horizontal width but smaller amplitude than those seen in normal rats appeared in layer IV. Correspondingly, neither cytochrome oxidase staining nor biotinylated dextran amine labeling of thalamocortical axons showed any barrel-like clusters or glomerular axon terminals. Likewise, at P5-P6, the tangential width of clusters in layer IV were larger than that in normal rats. At P10 in P7-lesioned rats, small cluster-matched barrels were seen in the optical response as well as in normal rats. These results suggest that P0 infraorbital nerve cut interrupted segregation of functional synapses into the barrels and retarded the maturation of thalamocortical transmission.

Activity-dependent regulation of GABAA receptors

GABAA receptor heterogeneity is based on the combinatorial assembly of a large family of subunits into distinct receptor subtypes. A neuron-specific expression pattern of receptor subtypes has been demonstrated in adult rat brain, which can be reproduced in vitro in primary neuron cultures. This suggests that genetic programs established during ontogeny govern the expression of gamma-aminobutyric acid (GABAA) receptor subtypes. Activity-dependent mechanisms nevertheless modulate on a short-term basis the cell surface expression of GABAA receptors, as demonstrated in cultured hippocampal neurons upon blockade of synaptic transmission or application of brain-derived neurotrophic factor. Preliminary evidence points to changes in protein phosphorylation as a mechanism underlying short-term activity-dependent regulation of GABAA receptors. In vivo, chronic pharmacological modulation of neuronal activity during development, while having marked effects on the rate of cortical growth, failed to influence the expression of GABAA receptor subtypes, suggesting that additional factors are involved.

Maturation of NADPH-d activity in the rat's barrel-field cortex and its relationship to cytochrome oxidase activity

Histochemical detection of NADPH-d activity in rat barrel-field cortex reveals four types of distributions. (i) A transient, diffuse neuropil staining is visible in the cortical plate and in deeper layers until postnatal day (P) 4. Thereafter, until P15, it is segregated in whisker-specific patches in layer IV, then the pattern gradually disappears, becoming virtually indistinct by P21. This transient patterning of diffuse NADPH-d activity in layer IV disappears after cortical injections of kainic acid and is affected by neonatal damage to the contralateral snout. An intense labeling (ii) of scattered cells and (iii) of a plexus of fibers is present. With maturation, the cells become localized mostly in layers II/III, in the lower part of layer V, and in layer VI. They are sparse in layer I, in upper layer V, and in layer IV where their somata are located primarily in the interbarrel septa. (iv) Light staining of cortical neurons is detected mostly in layers II-IV but occasionally also in layers V-VI. Cytochrome c oxidase (CO)-positive patches associated with barrels are first detected in layer IV around P4-P5; their staining density increases with development, then stays high. In the adult, CO activity is moderate in supragranular layers, highest in the barrels in layer IV, low in upper layer V, medium dense in the deeper half of layer V, and low in lamina VI. Thus, NADPH-d and CO activities are not necessarily colocalized in the rodent barrel-field cortex. The varied (transient and long-lasting) distributions of NADPH-d activity indicate that the enzyme and its associated production of NO serve multiple roles in developing and adult barrel-field cortex.

Primary cortical glial reaction versus secondary thalamic glial response in the excitotoxically injured young brain: microglial/macrophage response and major histocompatibility complex class I and II expression

The excitatory amino acid analog, N-methyl-D-aspartate, was injected intracortically into nine-day-old rats. Resulting axon-sparing lesions in the developing sensorimotor cortex, which secondarily affect thalamic neurons that become deprived of cortical targets, provide an experimental model for the study of the glial response in distantly affected areas. The microglial/macrophage response was studied using tomato lectin histochemistry and major histocompatibility complex I and II immunocytochemistry. Blood-brain barrier integrity was evaluated. In the cortical lesion site, where blood-brain barrier breakdown occurs, the rapid microglial response was restricted to the degenerating area. Microglial changes were first seen at 4 h post-injection, peaking at days 3-5. Reactive microglia changed morphology, increased tomato lectin binding and expressed major histocompatibility complex I. Additionally, some cells expressed major histocompatibility complex II. In the secondarily affected thalamus, the microglial response was not as pronounced as in the cortex, was first seen at 10 h post-injection and peaked at days 3-5. Reactive microglia showed a bushy morphology, were intensely lectin positive and expressed major histocompatibility complex I. The exceptional response of the nine-day-old brain to cortical lesions makes this model an interesting tool for studying the implications of microglial major histocompatibility factor expression in still enigmatic processes such as wound healing and plasticity.

Infraorbital nerve transection alters serotonin transporter expression in sensory pathways in early postnatal rat development

The serotonin transporter MRNA has been found throughout the trigeminal sensory system late in gestation and during early postnatal development, a period known to be critical for maturation of the sensory circuitry. The purpose of the present study was to determine whether sensory denervation in newborn rat pups would alter either the density or pattern of expression of the 5-HT transporter (5-HTT) within the trigeminal system. We combined autoradiographic localization of 5-HT transporters and in situ hybridization techniques to visualize both the transporter protein and mRNA in thalamic sensory neurons and in the somatosensory cortex following unilateral infraorbital nerve transection at postnatal day 1. For comparative purposes, similar measurements were conducted in thalamic visual neurons as well as in the visual cortex. Lesion of the infraorbital nerve decreased the [3H]citalopram labelling of 5-HT transporters in the ventral basal and ventral medial areas of the thalamus contralateral to the lesion, while labelling of 5-HT transporters was decreased in both contralateral and ipsilateral sides of the lateral genicuate (visual thalamus). Citalopram labelling of 5-HT transporters was not significantly altered in somatosensory or in cingulate cortex, however a significant decrease was observed in the visual cortex. In contrast, there were no obvious changes in the intensity of the 5-HT mRNA hybridization signal in sensory or visual thalamic areas. Given that the serotonin transporter regulates extracellular concentrations of 5-HT, the present data suggest that altered peripheral innervation and thereby altered sensory inputs to the thalamus during fetal development can potentially influence 5-HT transporter densities and thus, may influence extracellular levels of 5-HT in thalamus and cortex during a critical period of synapse formation. In turn, modulation of 5-HT transporter levels may influence extracellular concentrations of 5-HT in thalamus and cortex during a critical period of synapse formation.

Effect of gravity on vestibular neural development

The timing, molecular basis, and morphophysiological and behavioral consequences of the interaction between external environment and the internal genetic pool that shapes the nervous system over a lifetime remain important questions in basic neuroscientific research. Space station offers the opportunity to study this interaction over several life cycles in a variety of organisms. This short review considers past work in altered gravity, particularly on the vestibular system, as the basis for proposing future research on space station, and discusses the equipment necessary to achieve goals. It is stressed that, in keeping with the international investment being made in this research endeavor, both the questions asked and the technologies to be developed should be bold. Advantage must be taken of this unique research environment to expand the frontiers of neuroscience.

A role for tectal midline glia in the unilateral containment of retinocollicular axons

Retinal fibers approach close to the tectal midline but do not encroach on the other side. Just before the entry of retinal axons into the superior colliculus (SC), a group of radial glia differentiates at the tectal midline; the spatiotemporal deployment of these cells points to their involvement in the unilateral containment of retinotectal axons. To test for such a barrier function of the tectal midline cells, we used two lesion paradigms for disrupting their radial processes in the neonatal hamster: (1) a heat lesion was used to destroy the superficial layers of the right SC, including the midline region, and (2) a horizontally oriented hooked wire was inserted from the lateral edge of the left SC toward the midline and was used to undercut the midline cells, leaving intact the retinorecipient layers in the right SC. In both cases, the left SC was denervated by removing its contralateral retinal input. Animals were killed 12 hr to 2 weeks later, after intraocular injections of anterograde tracers to label the axons from the remaining eye. Both lesions resulted in degeneration of the distal processes of the tectal raphe glia and in an abnormal crossing of the tectal midline by retinal axons, leading to an innervation of the opposite ("wrong") tectum. The crossover occurred only where glial cell attachments were disrupted. These results document that during normal development, the integrity of the midline septum is critical in compartmentalizing retinal axons and in retaining the laterality of the retinotectal projection.

BDNF and NT-3 applied in the whisker pad reverse cortical changes after peripheral deafferentation in neonatal rats

It has been known for a long time that subcortical input drives the specification of cortical areas. Molecular signals mediating this instructive effect from the periphery are poorly understood. In foetal or neonatal rats, ablation of whisker follicles, transection of the infraorbital nerve, inhibition of axonal transport, but not impulse activity blockade, prevent formation of barrels in the primary somatosensory cortex (S1). These findings suggest that a chemical signal, possibly arising from the skin or the follicle, may be responsible for somatotopic pattern formation in S1. Neurotrophins promote survival and differentiation of primary sensory neurons, and are expressed in the whisker pad during development. Neonatal rats received gelfoam impregnated with NGF, BDNF or NT-3 under the whisker pad following surgical denervation of whisker rows D and E on P0. Barrel formation in S1 was assessed on P7 by acetylcholinesterase histochemistry and 5-HT-immunohistochemistry. BDNF and NT-3, but not NGF, promoted development of the cortical barrels corresponding to denervated whiskers. Furthermore, BDNF and NT-3 prevented the lesion-induced expansion of row C barrels, while NGF appeared to promote row C expansion. Our results suggest that BDNF and NT-3 arising from the whisker pad are involved in the formation and/or maintenance of the barrel pattern in S1. These findings are potentially relevant for the prevention of sensory disturbances possibly due to reorganization of central sensory circuits after peripheral nerve lesions in humans.

Development of whisker-related patterns in marsupials: factors controlling timing

In mature rodents, whisker-related patterns are known to be present in three levels of the brain: the brainstem trigeminal nuclei, the ventrobasal thalamus and the somatosensory cortex. These patterns have been demonstrated using neuroanatomical tracing techniques, histological and histochemical staining methods and electrophysiological recordings. The development and topography of these patterns are dependent on an intact periphery. But what governs when patterns form at the three levels? Possibilities include a controlling signal from the periphery or local mechanisms at each site, such as the arrival of afferent inputs or the maturation of target tissue. In this review, we report on the maturation of the whisker pathway in a marsupial, the wallaby, where the slow tempo of development is a feature. At each level, afferent fibres grow into the region of termination many weeks before the whisker-related pattern emerges. The results suggest that the maturity of the target tissue as well as signals from the periphery combine to trigger pattern formation at each level of the pathway.

Effects of monoamine oxidase A inhibition on barrel formation in the mouse somatosensory cortex: determination of a sensitive developmental period

Genetic inactivation of monoamine oxidase A (MAOA) in C3H/HeJ mice causes a complete absence of barrels in the somatosensory cortex, and similar alterations are caused by pharmacological inhibition of MAOA in wild type mice. To determine when and how MAOA inhibition affects the development of the barrel field, the MAOA inhibitor clorgyline was administered to mice of the outbred strain OF1 for various time periods between embryonic day 15 (E15) and postnatal day 7 (P7), and the barrel fields were analyzed with cytochrome oxidase and Nissl stains in P10 and adult mice. High-pressure liquid chromatography measures of brain serotonin (5-HT) showed three- to eightfold increases during the periods of clorgyline administration. Perinatal mortality was increased and weight gain was slowed between P3 and P6. Clorgyline treatments from E15 to P7 or from P0 to P7 disrupted the formation of barrels in the anterior snout representation and in parts of the posteromedial barrel subfield (PMBSF). Treatments from P0 to P4 caused similar although less severe barrel field alterations. Clorgyline treatments only during embryonic life or starting on P4 caused no detectable abnormalities. In cases with barrel field alterations, a rostral-to-caudal gradient of changes was noted: Rostral barrels of the PMBSF were most frequently fused and displayed an increased size tangentially. Thus, MAOA inhibition resulting in increased brain levels of 5-HT affects barrel development during the entire first postnatal week, with a sensitive period between P0 and P4. The rostral-to-caudal gradient of changes in the barrel field parallels known developmental gradients in the sensory periphery and in the maturation thalamocortical afferents. The observed barrel fusions could correspond to a default in the initial segregation of thalamic fibers or to a continued, exuberant growth of these fibers that overrides the tangential domain that is normally devoted to individual whiskers.

Phospholipase C-beta1 expression correlates with neuronal differentiation and synaptic plasticity in rat somatosensory cortex

Receptor-mediated signal transduction is thought to play an important role in neuronal differentiation and the modification of synaptic connections during brain development. The intracellular signalling molecule phospholipase C-beta1 (PLC-beta1), which is activated via specific neurotransmitter receptors, has recently been implicated in activity-dependent plasticity in the cat visual cortex. PLC-beta1 has been shown to be concentrated in an intermediate compartment-like organelle, the botrysome, which is present in 5-week-old, but not adult, cat cortical neurons. We have characterized the spatial and temporal regulation of PLC-beta1 expression in the developing rat cerebral cortex. PLC-beta1-positive botrysome-like organelles are observed during early postnatal cortical development, but not at postnatal day 14 or later stages. In the postnatal somatosensory cortex, there is also striking spatial variation in diffuse neuropilar immunoreactivity of layer IV and above, in a pattern corresponding to the thalamocortical recipient zones known as barrels. This expression pattern is specific to the developing barrel field and is most distinct at postnatal days 4-7, when cellular components of barrels are capable of activity-dependent modification. During later stages of cortical maturation, stained botrysomes disappear, expression of PLC-beta1 is down-regulated and only diffuse immunoreactivity remains in dendritic processes. Our results are consistent with a role for PLC-beta1 in activity-dependent, receptor-mediated neuronal plasticity during development of the somatosensory cortex.

Developmental changes in the electrophysiological properties of brain stem trigeminal neurons during pattern (barrelette) formation

In the brain stem trigeminal nuclei of rodents there is a patterned representation of whiskers and sinus hairs. The subnucleus interpolaris (SPI) contains the largest and the most conspicuous whisker patterns (barrelettes). Although neural activity plays a role in pattern formation, little is known about the electrophysiological properties of developing barrelette neurons. Here we examined the functional state of early postnatal SPI neurons during and after the consolidation of patterns by using in vitro intracellular recording techniques. After the consolidation of barrelettes [>/= postnatal day (P)4], responses to intracellular current injection consistently reflected the activation of a number voltage-dependent conductances. Most notable was a mixed cation conductance (IH) that prevented strong hyperpolarization and a large low-threshold Ca2+ conductance, which led to Ca2+ spikes and burst firing. At the oldest ages tested (P11-P14) some cells also exhibited an outward K+ conductance (IA), which led to significant delays in action-potential firing. Between P0-3, a time when the formation of barrelettes in the brain stem is still susceptible to damage of the sensory periphery, cells responded linearly to intracellular current injection, indicating they either lacked such voltage-gated properties or weakly expressed them. At all ages tested (P0-14), SPI cells were capable of generating trains of action potentials in response to intracellular injection of depolarizing current pulses. However, during the first few days of postnatal life, spikes were shorter and longer. Additionally, spike trains rose more linearly with stimulus intensity and showed frequency accommodation at early ages. Taken together, these results indicate that the electrophysiological properties of SPI neurons change markedly during the period of barrelette consolidation. Moreover, the properties of developing SPI neurons may play a significant role in pattern formation by minimizing signal distortion and ensuring that excitatory responses from sensory periphery are accurately received and transmitted according to stimulus strength.

Sensitive period for lesion-induced reorganization of intracortical projections within the vibrissae representation of rat's primary somatosensory cortex

Previous experiments from this laboratory demonstrated that intracortical connections in lamina IV of the rat primary somatosensory cortex (SI) are most dense outside the patches of cytochrome oxidase (CO) staining that correspond to the mystacial vibrissae. This pattern of intracortical connections becomes apparent on postnatal day 4 (P-4), at least 2 days after the appearance of the vibrissae-related pattern of thalamocortical afferents. Transection of the infraorbital nerve (ION) on the day of birth (P-0) disrupts both the CO and intracortical projection patterns. This series of experiments was undertaken to determine whether the patterning of either thalamocortical afferents or intracortical projections defines the end of the period over which peripheral damage can alter intracortical projections in lamina IV of SI. The infraorbital nerve (ION) was transected in different cohorts of rats on P-1 through P-5, and animals were allowed to survive > or =45 days, at which time biotinylated dextran amine (BDA) injections were made into the SI. After 7 days, animals were killed, and alternate cortical sections were processed for the demonstration of BDA or CO. Transection of the ION on P-1 or P-2 altered the patterning of both CO and intracortical connections in the SI. In contrast, cutting the ION on P-3 left the pattern of CO densities in the SI intact, but significantly altered the patterning of intracortical connections. Transection of the nerve on P-5 resulted in qualitatively and quantitatively normal patterns of both CO densities and BDA-labelled intracortical projections. These results indicate that the establishment of a stable barrel pattern in layer IV of the SI is not sufficient for normal adult patterning of intracortical projections in this lamina. However, once the mature pattern of intracortical projections in layer IV is established, ION lesions can no longer alter it.

NMDA receptor-dependent refinement of somatotopic maps

We have examined the role of NMDA receptor-mediated neural activity in the formation of periphery-related somatosensory patterns, using genetically engineered mice. We demonstrate that ectopic expression of a transgene of an NMDAR1 splice variant rescues neonatally fatal NMDAR1 knockout (KO) mice, although the average life span varies depending on the level of the transgene expression. In NMDAR1 KO mice with "high" levels of the transgene expression, sensory periphery-related patterns were normal along both the trigeminal and dorsal column pathways. In the KO mice with "low" levels of the transgene expression, the patterns were absent in the trigeminal pathway. Our results indicate that NMDA receptor-mediated neural activity plays a critical role in pattern formation along the ascending somatosensory pathways.

Fast synaptic signaling by nicotinic acetylcholine and serotonin 5-HT3 receptors in developing visual cortex

Cholinergic and serotonergic fiber systems invade the developing visual cortex several weeks before eye opening; both transmitters have been implicated in plasticity of neocortical circuits. These transmitters have been presumed to act predominantly through second messenger-coupled receptors, because fast cholinergic or serotonergic neurotransmission has never been observed in neocortex. However, acetylcholine and serotonin also act on ligand-gated ion channels; the nicotinic acetylcholine receptor and the serotonin 5-HT3 receptor, respectively. Here, using whole-cell patch-clamp techniques in developing ferret visual cortex, we pharmacologically isolated fast, spontaneous, and evoked cholinergic and serotonergic synaptic events in pyramidal cells and interneurons of all cortical layers. The number of cells receiving such inputs increased with the ingrowth of thalamic afferents, and the frequencies of the spontaneous events increased at eye opening. Thus, both acetylcholine and serotonin can mediate fast synaptic transmission in the visual cortex; the early onset of these mechanisms suggests a role during initial stages of circuit formation and during subsequent experience-dependent remodeling of cortical connections.

Timecourse of development of the wallaby trigeminal pathway: III. Thalamocortical and corticothalamic projections

The development of trigeminal projections between the thalamus and cortex has been investigated in the marsupial mammal, the wallaby, by using a carbocyanine dye, horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), Neurobiotin, and biocytin as pathway tracers. The appearance of whisker-related patterns in the cortex in relation to their appearance in the brainstem and thalamus was examined, as was the presence or absence of a waiting period for thalamocortical afferents and the identity of the first cortical cells to project to the thalamus. Thalamic afferents first reached the cortex at postnatal day (P) 15 and were distributed up to the deep edge of the compact cell zone in the superficial cortical plate throughout development, in both dye and WGA-HRP labelled material, with no evidence of a waiting period. The initial corticothalamic projection, detected by retrograde transport of WGA-HRP from the thalamus, occurred at P60 from layer 5 cells. This was confirmed by labelling of corticothalamic axons after cortical injections of Neurobiotin and biocytin. Scattered, labelled cells seen before P60 after dye labelling from the thalamus presumably resulted from transcellular labelling via thalamic afferents. Clustering of afferents in layer 4 and cell bodies and their dendrites in layers 5 and 6 first occurred simultaneously at P76. There is a sequential onset of pattern formation, first in brainstem, then in thalamus, and finally in cortex, with a long delay between afferent arrival and pattern formation at each level. Independent regulation at each level, likely depending on target maturation, is suggested.

Functional development of the vibrissae somatosensory system of the rat: (14C) 2-deoxyglucose metabolic mapping study

Functional development of the rat whisker somatosensory system was studied by using the (14C) 2-deoxyglucose (2DG) metabolic mapping technique. Restrained rat pups had their left mystacial vibrissae stroked for 30 minutes and their brains harvested, sectioned, and autoradiographed from the level of the lower medulla to the frontal cortex. Subjects were tested at postnatal days (PNDs) 0-9 and 21. At birth, all subjects exhibited a significant increase of 2DG uptake in the left spinal trigeminal nuclei, the principal trigeminal sensory nucleus, and a portion of the right ventral posteromedial thalamic nucleus. The primary somatosensory cortex exhibited significant 2DG uptake contralateral to stimulation by PND 6, followed by the secondary somatosensory cortex at PND 7. The pattern of 2DG uptake in the somatosensory cortices became more intense and well defined by PND 9. Given that the somatosensory system develops in an orderly fashion from the periphery to higher brain structures, the present results show that brain structures mediating whisker sensory input are not metabolically active until projections from lower somatosensory centers are established. Neurons become responsive to whisker stimulation in the subcortical structures at birth and in the somatosensory cortex a few days later. This cortical activity follows the organization of the upper tier of thalamocortical fibers into a "barrelfield." Moreover, there is a gradual enhancement in functional activity of the vibrissa neurons at different somatosensory nuclei as rats mature. The present study elucidates the time course of functional development in the rat somatosensory system.

Mutations of the homeobox genes Dlx-1 and Dlx-2 disrupt the striatal subventricular zone and differentiation of late born striatal neurons

The striatum has a central role in many neurobiological processes, yet little is known about the molecular control of its development. Inroads to this subject have been made, due to the discovery of transcription factors, such as the Dlx genes, whose expression patterns suggest that they have a role in striatal development. We report that mice lacking both Dlx-1 and Dlx-2 have a time-dependent block in striatal differentiation. In these mutants, early born neurons migrate into a striatum-like region, which is enriched for markers of the striosome (patch) compartment. However, later born neurons accumulate within the proliferative zone. Several lines of evidence suggest that mutations in Dlx-1 and Dlx-2 produce abnormalities in the development of the striatal subventricular zone and in the differentiation of striatal matrix neurons.

Area-specific regulation of gamma-aminobutyric acid type A receptor subtypes by thalamic afferents in developing rat neocortex

Targeting and innervation of the cerebral cortex by thalamic afferents is a key event in the specification of cortical areas. The molecular targets of thalamic regulation, however, have remained elusive. We now demonstrate that thalamic afferents regulate the expression of gamma-aminobutyric acid type A (GABAA) receptors in developing rat neocortex, leading to the area-specific expression of receptor subtypes in the primary visual (V1) and somatosensory (S1) areas. Most strikingly, the alpha1- and alpha5-GABAA receptors exhibited a reciprocal expression pattern, which precisely reflected the distribution of thalamocortical afferents at postnatal day 7. Following unilateral lesions at the birth of the thalamic nuclei innervating V1 and S1 (lateral geniculate nucleus and ventrobasal complex, respectively), profound changes in subunit expression were detected 1 week later in the deprived cortical territories (layers III-IV of V1 and S1). The expression of the alpha1 subunit was strongly down-regulated in these layers to a level comparable to that in neighboring areas. Conversely, the alpha5 subunit was up-regulated and areal boundaries were no longer discernible in the lesioned hemisphere. Changes similar to the alpha5 subunit were also seen for the alpha2 and alpha3 subunits. These results indicate that the differential expression of GABAA receptor subtypes in developing neocortex is dependent on thalamic innervation, contributing to the emergence of functionally distinct areas.

Developmental expression of keratan sulfate-like immunoreactivity distinguishes thalamic nuclei and cortical domains

Proteoglycans influence axonal outgrowth in several experimental paradigms, and their distribution during development suggests a role in axon guidance. We have used a monoclonal antibody, 5D4, that recognizes an epitope on sulfated keratans (KS), to define the distribution of keratan sulfate proteoglycans (KSPGs) in the developing thalamus and cortex of the rat. During development, 5D4 immunolabeling is present on thalamic axons as they grow through the internal capsule and subplate but is not present in the adjacent pathway for cortical efferent axons. Individual thalamic nuclei differ markedly in their expression of KSPGs; these distinctions persist throughout the period of developmentally regulated expression. Major cortical domains also differ in their expression of KSPGs, which are expressed throughout medial (cingulate and retrosplenial) cortex well before neocortex. Immunolabeling for KSPGs diminishes 2 weeks after birth; in the adult it is associated with small glia. The 5D4 epitope is present on several KSPGs (320, 220, and 160 kD) on Western blots during development but only in a broad 200-kD band in adult brain. Immunolabeling is degraded on sections and Western blots by keratanase II but not by keratanase I or chondroitinase ABC, confirming that the antibody recognizes KS. Bands identified by 5D4 on Western blots differ from those identified by antibodies to known KSPGs (aggrecan, claustrin, SV2, ABAKAN, phosphacan-KS), indicating that 5D4 is labeling KSPGs not previously described in the brain. The selective expression of KSPGs during development suggests that they may be a part of the molecular identity of thalamic nuclei and cortical domains that defines their connectivity.

The role of activity in the development of long-range horizontal connections in area 17 of the ferret

Horizontal connections in area 17 of adult cats and ferrets link cells with similar preferred orientations by a patchy network of projections extending several millimeters across the cortex. The maturation of orientation selectivity in ferret area 17 has been demonstrated previously by quantitative single-unit recording and optical imaging to begin at approximately postnatal days (P) 32-P36. We therefore made restricted injections of cholera toxin B-subunit (CTB) or CTB-gold into ferret area 17 at a series of developmental ages and statistically quantified the degree of clustering in plots of retrogradely labeled cells in tangential sections through layer III for comparison to the published values for orientation tuning at each age. At P21, horizontal connections within area 17 lacked patchiness entirely, although clear patches of labeled cells were present in extrastriate areas. By P27, significant clustering of horizontal connections within area 17 was present. A second phase of cluster refinement was observed to occur at approximately P34-P36, coinciding with the emergence of mature orientation tuning and maps. Continuous silencing of cortical action potentials by chronic tetrodotoxin infusion from P21 resulted in a spatially random distribution of retrogradely labeled cells at P34. In contrast, bilateral enucleation from P21 did not prevent the initial development of clustered horizontal connections. We conclude, based on our findings and those of others, that the anatomical specificity of long-range horizontal connections results from an activity-dependent process that initially can use spontaneous activity in the cortical and thalamic networks to establish crude periodic connections and later uses visual cues to refine these connections.

Development of local connections in ferret somatosensory cortex

Ferrets have become recognized as a useful and interesting model for study of neocortical development. Because of their immaturity at birth, it is possible to study very early events in the ontogeny of the brain. We used living slices of ferret somatosensory cortex to study the formation and development of intrinsic elements within the neocortex. A small number of fixed, hemisected brains injected with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) were also used. The slices were obtained from ferret kits aged postnatal day (P)1 to P62 and maintained in a chamber; each slice received injections of fluorescent-labeled dextrans. The injections were made at different ages in several distinct sites, which included the proliferative ventricular zone, the intervening white matter (or intermediate zone), and different sites of developing cortex, including the deeper cortical plate, which incorporated the subplate in young animals, and more superficial cortical sites depending on the age of the animal. Several animals also received injections into the ventrobasal thalamus. Injections into young animals (P1-7) produced a dominant radial pattern that extended from the ventricular zone into the cortex. Injections into the ventricular zone labeled many cells that appeared morphologically like radial glia as well as presumptive neurons. Although the predominant pattern was radial, injections in the ventricular zone often produced tangentially oriented cells and horizontally arranged fibers at the outer edge of the proliferative zone. These cells and fibers may provide a substrate for tangential dispersion of neurons within the neocortex. More superficial injections within the slice labeled lines of cells that appeared to be stacked upon one another in a radial pile in the cortex; the cortical plate received very few lateral projections. Data obtained from more mature slices indicated that although the overall pattern of staining remained radial, the precise character of the pattern changed to include more lateral spread into surrounding cortex, which eventually refined and developed into distinct patches by P28, when the overall cortical architecture appeared adult like. The data involving thalamocortical connections were more limited, but they indicated that the thalamus projects precisely to the somatosensory cortex in a point-to-point fashion from the earliest date studied (P0) and that the ventrobasal nucleus terminates upon the somatosensory cortex in a patchy manner during the early postnatal days of development. This study of the development of the somatosensory cortex confirms the ubiquitous nature of column-like connections throughout the neocortex and provides a novel view of the radial nature of early neocortical maturation.